From first principles, a point force applied to a pistonic loudspeaker diaphragm will provide a naturally flat frequency response but a power response which falls at higher frequencies. This is due to the radiation coupling changing as the radiated wavelength becomes comparable with the length l of the diaphragm, or the half diameter or radius a for a circular diaphragm, i.e. where ka is greater than 2 or kl is greater than 4 (k is the wave number frequency). However for a theoretical, free mounted bending wave panel speaker, a pure force, i.e. mass-less point drive, will provide both flat sound pressure and flat sound power with frequency.
A practical bending wave panel will however be supported on a suspension, and have an exciter with a complex driving point impedance including a mass. Such an object will demonstrate an uneven frequency response compared with the theoretical expectation. This is due to the various masses and compliances now present unbalancing the panel's modal behaviour. Where the modal density is high enough, the system may be designed so that the modes are beneficially distributed over frequency for a more even acoustic response. But this distributed mode method may not be so effective at the lower bending frequencies where modes are sparse and generally insufficient to construct a satisfactory frequency response.
The objective of flat pressure and power response down to the lowest bending frequency, so bridging the gap to the pistonic or whole body range, requires that the theoretical condition of modal balance be re-established. If this can be achieved, the adjusted modal balance restores the acoustic action of the practical panel to the desired theoretical condition. This would provide a new class of loudspeaker radiator and where the radiated response, in terms of power or frequency, is independent of drive point mass.
The goal for the designer of transducers and loudspeakers employing practical diaphragms and drive methods is to obtain an operation essentially independent of frequency. Once that primary objective is realised, other desired characteristics may be engineered by the designer.